Polysilicon is a basic raw material for the photovoltaic and semiconductor industries, and demand thereof has dramatically increased along with the development of its related industrial fields in recent years.
Polysilicon is mainly manufactured by a method of precipitating a silicon element on a silicon surface through thermal decomposition and/or a hydrogen reduction reaction of a silicon-containing source gas, and representative examples thereof include a method using a bell-jar type of reactor (also called Siemens method) and a method using a fluidized bed reactor.
Among them, the Siemens method is a traditional method of depositing silicon on the surface of a silicon rod provided in a bell-jar reactor. In the Siemens process, the surface area required for the deposition of silicon is limited and there is a limitation to the diameter of the silicon rod which is increased by the precipitation reaction, so a continuous process is impossible. In addition, the Siemens method has a limitation in productivity because the power consumption per unit weight of polysilicon to be produced is high.
The method using a fluidized bed reactor is a method of obtaining granular polysilicon by injecting a silicon source gas into a fluidized bed of a silicon seed which is heated and flowing at a high temperature, and depositing silicon on the surface of the seed. In the method using a fluidized bed reactor, the surface area of the seed in which deposition of silicon can occur is wide, the deposition of silicon can be performed at a relatively low temperature, and the post-treatment process is simple, such productivity is high compared to the Siemens method.
In the method using a fluidized bed reactor, the pattern of the fluidized bed is largely divided into a bubbling fluidized bed and a spout fluidized bed.
In the bubbling fluidized bed, small bubbles are uniformly distributed throughout the fluidized bed to form a constant turbulent flow pattern, which is advantageous for material and heat transfer. Such a method utilizing a bubbling fluidized bed is suitable for obtaining products having a uniform particle size and high purity with a low speed and low concentration operation, and internal abrasion damage to the reactor is not severe. However, in the case of the bubbling fluidized bed, since momentum of the supplied reaction gas is not large, it is difficult to cope with a layer separation phenomenon which occurs when the particles grow to a certain size or more. Therefore, the method using the bubbling fluidized bed has a limitation that the discharge of the product and the replenishment of the seed must be performed in a short cycle.
The spout fluidized bed shows a flow pattern in which a high-speed and high-concentration reaction gas is intensively injected from the center of the reactor, particles from the central portion ascend, and particles from the outer portion descend. In such a spout fluidized bed, since it is advantageous in coping with formation of particle agglomerates and the generation of fine particles, the concentration of the silicon source gas can be set to be higher than that during the operation of the bubbling fluidized bed. In the spout fluidized bed, since a gas having high momentum is supplied, the non-fluidization phenomenon does not easily occur even if resistance increases. In addition, in the spout fluidized bed, in order to prevent high-speed reaction gas from passing through the fluidized bed without reaction, the height of the fluidized bed is set to be higher than that of the bubbling fluidized bed.
However, an increase in the height of the fluidized bed means an increase in the height of the reactor, and the abrasion damage to the inside of the reactor may become severe due to high-speed particle flow. Therefore, the method using the spout fluidized bed has a disadvantage in that the installation and maintenance costs of the reactor are high. Further, when the size of the seeds in the spout fluidized bed is small, an entrainment phenomenon due to a strong flow velocity may occur, so that there is a restriction in using seeds exceeding a certain size.
Thus, in the method using the fluidized bed reactor, the process stability and the productivity are in a trade-off relationship with each other depending on the pattern of the fluidized bed, so there is an urgent need for a method and means capable of simultaneously improving the stability and productivity of the polysilicon preparing process.